/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.io.Serializable;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.util.Collection;
import java.util.List;
import java.util.RandomAccess;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Abstract base class for tasks that run within a {@link ForkJoinPool}.
 * A {@code ForkJoinTask} is a thread-like entity that is much
 * lighter weight than a normal thread. Huge numbers of tasks and
 * subtasks may be hosted by a small number of actual threads in a
 * ForkJoinPool, at the price of some usage limitations.
 *
 * <p>A "main" {@code ForkJoinTask} begins execution when it is
 * explicitly submitted to a {@link ForkJoinPool}, or, if not already
 * engaged in a ForkJoin computation, commenced in the {@link
 * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
 * related methods. Once started, it will usually in turn start other
 * subtasks. As indicated by the name of this class, many programs
 * using {@code ForkJoinTask} employ only methods {@link #fork} and
 * {@link #join}, or derivatives such as {@link
 * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
 * provides a number of other methods that can come into play in
 * advanced usages, as well as extension mechanics that allow support
 * of new forms of fork/join processing.
 *
 * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
 * The efficiency of {@code ForkJoinTask}s stems from a set of
 * restrictions (that are only partially statically enforceable)
 * reflecting their main use as computational tasks calculating pure
 * functions or operating on purely isolated objects. The primary
 * coordination mechanisms are {@link #fork}, that arranges
 * asynchronous execution, and {@link #join}, that doesn't proceed
 * until the task's result has been computed. Computations should
 * ideally avoid {@code synchronized} methods or blocks, and should
 * minimize other blocking synchronization apart from joining other
 * tasks or using synchronizers such as Phasers that are advertised to
 * cooperate with fork/join scheduling. Subdividable tasks should also
 * not perform blocking I/O, and should ideally access variables that
 * are completely independent of those accessed by other running
 * tasks. These guidelines are loosely enforced by not permitting
 * checked exceptions such as {@code IOExceptions} to be
 * thrown. However, computations may still encounter unchecked
 * exceptions, that are rethrown to callers attempting to join
 * them. These exceptions may additionally include {@link
 * RejectedExecutionException} stemming from internal resource
 * exhaustion, such as failure to allocate internal task
 * queues. Rethrown exceptions behave in the same way as regular
 * exceptions, but, when possible, contain stack traces (as displayed
 * for example using {@code ex.printStackTrace()}) of both the thread
 * that initiated the computation as well as the thread actually
 * encountering the exception; minimally only the latter.
 *
 * <p>It is possible to define and use ForkJoinTasks that may block,
 * but doing so requires three further considerations: (1) Completion
 * of few if any <em>other</em> tasks should be dependent on a task
 * that blocks on external synchronization or I/O. Event-style async
 * tasks that are never joined (for example, those subclassing {@link
 * CountedCompleter}) often fall into this category. (2) To minimize
 * resource impact, tasks should be small; ideally performing only the
 * (possibly) blocking action. (3) Unless the {@link
 * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
 * blocked tasks is known to be less than the pool's {@link
 * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
 * enough threads will be available to ensure progress or good
 * performance.
 *
 * <p>The primary method for awaiting completion and extracting
 * results of a task is {@link #join}, but there are several variants:
 * The {@link Future#get} methods support interruptible and/or timed
 * waits for completion and report results using {@code Future}
 * conventions. Method {@link #invoke} is semantically
 * equivalent to {@code fork(); join()} but always attempts to begin
 * execution in the current thread. The "<em>quiet</em>" forms of
 * these methods do not extract results or report exceptions. These
 * may be useful when a set of tasks are being executed, and you need
 * to delay processing of results or exceptions until all complete.
 * Method {@code invokeAll} (available in multiple versions)
 * performs the most common form of parallel invocation: forking a set
 * of tasks and joining them all.
 *
 * <p>In the most typical usages, a fork-join pair act like a call
 * (fork) and return (join) from a parallel recursive function. As is
 * the case with other forms of recursive calls, returns (joins)
 * should be performed innermost-first. For example, {@code a.fork();
 * b.fork(); b.join(); a.join();} is likely to be substantially more
 * efficient than joining {@code a} before {@code b}.
 *
 * <p>The execution status of tasks may be queried at several levels
 * of detail: {@link #isDone} is true if a task completed in any way
 * (including the case where a task was cancelled without executing);
 * {@link #isCompletedNormally} is true if a task completed without
 * cancellation or encountering an exception; {@link #isCancelled} is
 * true if the task was cancelled (in which case {@link #getException}
 * returns a {@link CancellationException}); and
 * {@link #isCompletedAbnormally} is true if a task was either
 * cancelled or encountered an exception, in which case {@link
 * #getException} will return either the encountered exception or
 * {@link CancellationException}.
 *
 * <p>The ForkJoinTask class is not usually directly subclassed.
 * Instead, you subclass one of the abstract classes that support a
 * particular style of fork/join processing, typically {@link
 * RecursiveAction} for most computations that do not return results,
 * {@link RecursiveTask} for those that do, and {@link
 * CountedCompleter} for those in which completed actions trigger
 * other actions. Normally, a concrete ForkJoinTask subclass declares
 * fields comprising its parameters, established in a constructor, and
 * then defines a {@code compute} method that somehow uses the control
 * methods supplied by this base class.
 *
 * <p>Method {@link #join} and its variants are appropriate for use
 * only when completion dependencies are acyclic; that is, the
 * parallel computation can be described as a directed acyclic graph
 * (DAG). Otherwise, executions may encounter a form of deadlock as
 * tasks cyclically wait for each other. However, this framework
 * supports other methods and techniques (for example the use of
 * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
 * may be of use in constructing custom subclasses for problems that
 * are not statically structured as DAGs. To support such usages, a
 * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
 * value using {@link #setForkJoinTaskTag} or {@link
 * #compareAndSetForkJoinTaskTag} and checked using {@link
 * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
 * these {@code protected} methods or tags for any purpose, but they
 * may be of use in the construction of specialized subclasses. For
 * example, parallel graph traversals can use the supplied methods to
 * avoid revisiting nodes/tasks that have already been processed.
 * (Method names for tagging are bulky in part to encourage definition
 * of methods that reflect their usage patterns.)
 *
 * <p>Most base support methods are {@code final}, to prevent
 * overriding of implementations that are intrinsically tied to the
 * underlying lightweight task scheduling framework. Developers
 * creating new basic styles of fork/join processing should minimally
 * implement {@code protected} methods {@link #exec}, {@link
 * #setRawResult}, and {@link #getRawResult}, while also introducing
 * an abstract computational method that can be implemented in its
 * subclasses, possibly relying on other {@code protected} methods
 * provided by this class.
 *
 * <p>ForkJoinTasks should perform relatively small amounts of
 * computation. Large tasks should be split into smaller subtasks,
 * usually via recursive decomposition. As a very rough rule of thumb,
 * a task should perform more than 100 and less than 10000 basic
 * computational steps, and should avoid indefinite looping. If tasks
 * are too big, then parallelism cannot improve throughput. If too
 * small, then memory and internal task maintenance overhead may
 * overwhelm processing.
 *
 * <p>This class provides {@code adapt} methods for {@link Runnable}
 * and {@link Callable}, that may be of use when mixing execution of
 * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
 * of this form, consider using a pool constructed in <em>asyncMode</em>.
 *
 * <p>ForkJoinTasks are {@code Serializable}, which enables them to be
 * used in extensions such as remote execution frameworks. It is
 * sensible to serialize tasks only before or after, but not during,
 * execution. Serialization is not relied on during execution itself.
 *
 * @since 1.7
 * @author Doug Lea
 */
public abstract class ForkJoinTask<V> implements Future<V>, Serializable {

 /*
 * See the internal documentation of class ForkJoinPool for a
 * general implementation overview. ForkJoinTasks are mainly
 * responsible for maintaining their "status" field amidst relays
 * to methods in ForkJoinWorkerThread and ForkJoinPool.
 *
 * The methods of this class are more-or-less layered into
 * (1) basic status maintenance
 * (2) execution and awaiting completion
 * (3) user-level methods that additionally report results.
 * This is sometimes hard to see because this file orders exported
 * methods in a way that flows well in javadocs.
 */

 /**
 * The status field holds run control status bits packed into a
 * single int to ensure atomicity. Status is initially zero, and
 * takes on nonnegative values until completed, upon which it
 * holds (sign bit) DONE, possibly with ABNORMAL (cancelled or
 * exceptional) and THROWN (in which case an exception has been
 * stored). Tasks with dependent blocked waiting joiners have the
 * SIGNAL bit set. Completion of a task with SIGNAL set awakens
 * any waiters via notifyAll. (Waiters also help signal others
 * upon completion.)
 *
 * These control bits occupy only (some of) the upper half (16
 * bits) of status field. The lower bits are used for user-defined
 * tags.
 */
 volatile int status; // accessed directly by pool and workers

 private static final int DONE = 1 << 31; // must be negative
 private static final int ABNORMAL = 1 << 18; // set atomically with DONE
 private static final int THROWN = 1 << 17; // set atomically with ABNORMAL
 private static final int SIGNAL = 1 << 16; // true if joiner waiting
 private static final int SMASK = 0xffff; // short bits for tags

 static boolean isExceptionalStatus(int s) { // needed by subclasses
 return (s & THROWN) != 0;
 }

 /**
 * Sets DONE status and wakes up threads waiting to join this task.
 *
 * @return status on exit
 */
 private int setDone() {
 int s;
 if (((s = (int)STATUS.getAndBitwiseOr(this, DONE)) & SIGNAL) != 0)
 synchronized (this) { notifyAll(); }
 return s | DONE;
 }

 /**
 * Marks cancelled or exceptional completion unless already done.
 *
 * @param completion must be DONE | ABNORMAL, ORed with THROWN if exceptional
 * @return status on exit
 */
 private int abnormalCompletion(int completion) {
 for (int s, ns;;) {
 if ((s = status) < 0)
 return s;
 else if (STATUS.weakCompareAndSet(this, s, ns = s | completion)) {
 if ((s & SIGNAL) != 0)
 synchronized (this) { notifyAll(); }
 return ns;
 }
 }
 }

 /**
 * Primary execution method for stolen tasks. Unless done, calls
 * exec and records status if completed, but doesn't wait for
 * completion otherwise.
 *
 * @return status on exit from this method
 */
 final int doExec() {
 int s; boolean completed;
 if ((s = status) >= 0) {
 try {
 completed = exec();
 } catch (Throwable rex) {
 completed = false;
 s = setExceptionalCompletion(rex);
 }
 if (completed)
 s = setDone();
 }
 return s;
 }

 /**
 * If not done, sets SIGNAL status and performs Object.wait(timeout).
 * This task may or may not be done on exit. Ignores interrupts.
 *
 * @param timeout using Object.wait conventions.
 */
 final void internalWait(long timeout) {
 if ((int)STATUS.getAndBitwiseOr(this, SIGNAL) >= 0) {
 synchronized (this) {
 if (status >= 0)
 try { wait(timeout); } catch (InterruptedException ie) { }
 else
 notifyAll();
 }
 }
 }

 /**
 * Blocks a non-worker-thread until completion.
 * @return status upon completion
 */
 private int externalAwaitDone() {
 int s = tryExternalHelp();
 if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) {
 boolean interrupted = false;
 synchronized (this) {
 for (;;) {
 if ((s = status) >= 0) {
 try {
 wait(0L);
 } catch (InterruptedException ie) {
 interrupted = true;
 }
 }
 else {
 notifyAll();
 break;
 }
 }
 }
 if (interrupted)
 Thread.currentThread().interrupt();
 }
 return s;
 }

 /**
 * Blocks a non-worker-thread until completion or interruption.
 */
 private int externalInterruptibleAwaitDone() throws InterruptedException {
 int s = tryExternalHelp();
 if (s >= 0 && (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) {
 synchronized (this) {
 for (;;) {
 if ((s = status) >= 0)
 wait(0L);
 else {
 notifyAll();
 break;
 }
 }
 }
 }
 else if (Thread.interrupted())
 throw new InterruptedException();
 return s;
 }

 /**
 * Tries to help with tasks allowed for external callers.
 *
 * @return current status
 */
 private int tryExternalHelp() {
 int s;
 return ((s = status) < 0 ? s:
 (this instanceof CountedCompleter) ?
 ForkJoinPool.common.externalHelpComplete(
 (CountedCompleter<?>)this, 0) :
 ForkJoinPool.common.tryExternalUnpush(this) ?
 doExec() : 0);
 }

 /**
 * Implementation for join, get, quietlyJoin. Directly handles
 * only cases of already-completed, external wait, and
 * unfork+exec. Others are relayed to ForkJoinPool.awaitJoin.
 *
 * @return status upon completion
 */
 private int doJoin() {
 int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
 return (s = status) < 0 ? s :
 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 (w = (wt = (ForkJoinWorkerThread)t).workQueue).
 tryUnpush(this) && (s = doExec()) < 0 ? s :
 wt.pool.awaitJoin(w, this, 0L) :
 externalAwaitDone();
 }

 /**
 * Implementation for invoke, quietlyInvoke.
 *
 * @return status upon completion
 */
 private int doInvoke() {
 int s; Thread t; ForkJoinWorkerThread wt;
 return (s = doExec()) < 0 ? s :
 ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 (wt = (ForkJoinWorkerThread)t).pool.
 awaitJoin(wt.workQueue, this, 0L) :
 externalAwaitDone();
 }

 // Exception table support

 /**
 * Hash table of exceptions thrown by tasks, to enable reporting
 * by callers. Because exceptions are rare, we don't directly keep
 * them with task objects, but instead use a weak ref table. Note
 * that cancellation exceptions don't appear in the table, but are
 * instead recorded as status values.
 *
 * The exception table has a fixed capacity.
 */
 private static final ExceptionNode[] exceptionTable
 = new ExceptionNode[32];

 /** Lock protecting access to exceptionTable. */
 private static final ReentrantLock exceptionTableLock
 = new ReentrantLock();

 /** Reference queue of stale exceptionally completed tasks. */
 private static final ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue
 = new ReferenceQueue<>();

 /**
 * Key-value nodes for exception table. The chained hash table
 * uses identity comparisons, full locking, and weak references
 * for keys. The table has a fixed capacity because it only
 * maintains task exceptions long enough for joiners to access
 * them, so should never become very large for sustained
 * periods. However, since we do not know when the last joiner
 * completes, we must use weak references and expunge them. We do
 * so on each operation (hence full locking). Also, some thread in
 * any ForkJoinPool will call helpExpungeStaleExceptions when its
 * pool becomes isQuiescent.
 */
 static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
 final Throwable ex;
 ExceptionNode next;
 final long thrower; // use id not ref to avoid weak cycles
 final int hashCode; // store task hashCode before weak ref disappears
 ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next,
 ReferenceQueue<ForkJoinTask<?>> exceptionTableRefQueue) {
 super(task, exceptionTableRefQueue);
 this.ex = ex;
 this.next = next;
 this.thrower = Thread.currentThread().getId();
 this.hashCode = System.identityHashCode(task);
 }
 }

 /**
 * Records exception and sets status.
 *
 * @return status on exit
 */
 final int recordExceptionalCompletion(Throwable ex) {
 int s;
 if ((s = status) >= 0) {
 int h = System.identityHashCode(this);
 final ReentrantLock lock = exceptionTableLock;
 lock.lock();
 try {
 expungeStaleExceptions();
 ExceptionNode[] t = exceptionTable;
 int i = h & (t.length - 1);
 for (ExceptionNode e = t[i]; ; e = e.next) {
 if (e == null) {
 t[i] = new ExceptionNode(this, ex, t[i],
 exceptionTableRefQueue);
 break;
 }
 if (e.get() == this) // already present
 break;
 }
 } finally {
 lock.unlock();
 }
 s = abnormalCompletion(DONE | ABNORMAL | THROWN);
 }
 return s;
 }

 /**
 * Records exception and possibly propagates.
 *
 * @return status on exit
 */
 private int setExceptionalCompletion(Throwable ex) {
 int s = recordExceptionalCompletion(ex);
 if ((s & THROWN) != 0)
 internalPropagateException(ex);
 return s;
 }

 /**
 * Hook for exception propagation support for tasks with completers.
 */
 void internalPropagateException(Throwable ex) {
 }

 /**
 * Cancels, ignoring any exceptions thrown by cancel. Used during
 * worker and pool shutdown. Cancel is spec'ed not to throw any
 * exceptions, but if it does anyway, we have no recourse during
 * shutdown, so guard against this case.
 */
 static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
 if (t != null && t.status >= 0) {
 try {
 t.cancel(false);
 } catch (Throwable ignore) {
 }
 }
 }

 /**
 * Removes exception node and clears status.
 */
 private void clearExceptionalCompletion() {
 int h = System.identityHashCode(this);
 final ReentrantLock lock = exceptionTableLock;
 lock.lock();
 try {
 ExceptionNode[] t = exceptionTable;
 int i = h & (t.length - 1);
 ExceptionNode e = t[i];
 ExceptionNode pred = null;
 while (e != null) {
 ExceptionNode next = e.next;
 if (e.get() == this) {
 if (pred == null)
 t[i] = next;
 else
 pred.next = next;
 break;
 }
 pred = e;
 e = next;
 }
 expungeStaleExceptions();
 status = 0;
 } finally {
 lock.unlock();
 }
 }

 /**
 * Returns a rethrowable exception for this task, if available.
 * To provide accurate stack traces, if the exception was not
 * thrown by the current thread, we try to create a new exception
 * of the same type as the one thrown, but with the recorded
 * exception as its cause. If there is no such constructor, we
 * instead try to use a no-arg constructor, followed by initCause,
 * to the same effect. If none of these apply, or any fail due to
 * other exceptions, we return the recorded exception, which is
 * still correct, although it may contain a misleading stack
 * trace.
 *
 * @return the exception, or null if none
 */
 private Throwable getThrowableException() {
 int h = System.identityHashCode(this);
 ExceptionNode e;
 final ReentrantLock lock = exceptionTableLock;
 lock.lock();
 try {
 expungeStaleExceptions();
 ExceptionNode[] t = exceptionTable;
 e = t[h & (t.length - 1)];
 while (e != null && e.get() != this)
 e = e.next;
 } finally {
 lock.unlock();
 }
 Throwable ex;
 if (e == null || (ex = e.ex) == null)
 return null;
 if (e.thrower != Thread.currentThread().getId()) {
 try {
 Constructor<?> noArgCtor = null;
 // public ctors only
 for (Constructor<?> c : ex.getClass().getConstructors()) {
 Class<?>[] ps = c.getParameterTypes();
 if (ps.length == 0)
 noArgCtor = c;
 else if (ps.length == 1 && ps[0] == Throwable.class)
 return (Throwable)c.newInstance(ex);
 }
 if (noArgCtor != null) {
 Throwable wx = (Throwable)noArgCtor.newInstance();
 wx.initCause(ex);
 return wx;
 }
 } catch (Exception ignore) {
 }
 }
 return ex;
 }

 /**
 * Polls stale refs and removes them. Call only while holding lock.
 */
 private static void expungeStaleExceptions() {
 for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
 if (x instanceof ExceptionNode) {
 ExceptionNode[] t = exceptionTable;
 int i = ((ExceptionNode)x).hashCode & (t.length - 1);
 ExceptionNode e = t[i];
 ExceptionNode pred = null;
 while (e != null) {
 ExceptionNode next = e.next;
 if (e == x) {
 if (pred == null)
 t[i] = next;
 else
 pred.next = next;
 break;
 }
 pred = e;
 e = next;
 }
 }
 }
 }

 /**
 * If lock is available, polls stale refs and removes them.
 * Called from ForkJoinPool when pools become quiescent.
 */
 static final void helpExpungeStaleExceptions() {
 final ReentrantLock lock = exceptionTableLock;
 if (lock.tryLock()) {
 try {
 expungeStaleExceptions();
 } finally {
 lock.unlock();
 }
 }
 }

 /**
 * A version of "sneaky throw" to relay exceptions.
 */
 static void rethrow(Throwable ex) {
 ForkJoinTask.<RuntimeException>uncheckedThrow(ex);
 }

 /**
 * The sneaky part of sneaky throw, relying on generics
 * limitations to evade compiler complaints about rethrowing
 * unchecked exceptions.
 */
 @SuppressWarnings("unchecked") static <T extends Throwable>
 void uncheckedThrow(Throwable t) throws T {
 if (t != null)
 throw (T)t; // rely on vacuous cast
 else
 throw new Error("Unknown Exception");
 }

 /**
 * Throws exception, if any, associated with the given status.
 */
 private void reportException(int s) {
 rethrow((s & THROWN) != 0 ? getThrowableException() :
 new CancellationException());
 }

 // public methods

 /**
 * Arranges to asynchronously execute this task in the pool the
 * current task is running in, if applicable, or using the {@link
 * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}. While
 * it is not necessarily enforced, it is a usage error to fork a
 * task more than once unless it has completed and been
 * reinitialized. Subsequent modifications to the state of this
 * task or any data it operates on are not necessarily
 * consistently observable by any thread other than the one
 * executing it unless preceded by a call to {@link #join} or
 * related methods, or a call to {@link #isDone} returning {@code
 * true}.
 *
 * @return {@code this}, to simplify usage
 */
 public final ForkJoinTask<V> fork() {
 Thread t;
 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
 ((ForkJoinWorkerThread)t).workQueue.push(this);
 else
 ForkJoinPool.common.externalPush(this);
 return this;
 }

 /**
 * Returns the result of the computation when it
 * {@linkplain #isDone is done}.
 * This method differs from {@link #get()} in that abnormal
 * completion results in {@code RuntimeException} or {@code Error},
 * not {@code ExecutionException}, and that interrupts of the
 * calling thread do <em>not</em> cause the method to abruptly
 * return by throwing {@code InterruptedException}.
 *
 * @return the computed result
 */
 public final V join() {
 int s;
 if (((s = doJoin()) & ABNORMAL) != 0)
 reportException(s);
 return getRawResult();
 }

 /**
 * Commences performing this task, awaits its completion if
 * necessary, and returns its result, or throws an (unchecked)
 * {@code RuntimeException} or {@code Error} if the underlying
 * computation did so.
 *
 * @return the computed result
 */
 public final V invoke() {
 int s;
 if (((s = doInvoke()) & ABNORMAL) != 0)
 reportException(s);
 return getRawResult();
 }

 /**
 * Forks the given tasks, returning when {@code isDone} holds for
 * each task or an (unchecked) exception is encountered, in which
 * case the exception is rethrown. If more than one task
 * encounters an exception, then this method throws any one of
 * these exceptions. If any task encounters an exception, the
 * other may be cancelled. However, the execution status of
 * individual tasks is not guaranteed upon exceptional return. The
 * status of each task may be obtained using {@link
 * #getException()} and related methods to check if they have been
 * cancelled, completed normally or exceptionally, or left
 * unprocessed.
 *
 * @param t1 the first task
 * @param t2 the second task
 * @throws NullPointerException if any task is null
 */
 public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
 int s1, s2;
 t2.fork();
 if (((s1 = t1.doInvoke()) & ABNORMAL) != 0)
 t1.reportException(s1);
 if (((s2 = t2.doJoin()) & ABNORMAL) != 0)
 t2.reportException(s2);
 }

 /**
 * Forks the given tasks, returning when {@code isDone} holds for
 * each task or an (unchecked) exception is encountered, in which
 * case the exception is rethrown. If more than one task
 * encounters an exception, then this method throws any one of
 * these exceptions. If any task encounters an exception, others
 * may be cancelled. However, the execution status of individual
 * tasks is not guaranteed upon exceptional return. The status of
 * each task may be obtained using {@link #getException()} and
 * related methods to check if they have been cancelled, completed
 * normally or exceptionally, or left unprocessed.
 *
 * @param tasks the tasks
 * @throws NullPointerException if any task is null
 */
 public static void invokeAll(ForkJoinTask<?>... tasks) {
 Throwable ex = null;
 int last = tasks.length - 1;
 for (int i = last; i >= 0; --i) {
 ForkJoinTask<?> t = tasks[i];
 if (t == null) {
 if (ex == null)
 ex = new NullPointerException();
 }
 else if (i != 0)
 t.fork();
 else if ((t.doInvoke() & ABNORMAL) != 0 && ex == null)
 ex = t.getException();
 }
 for (int i = 1; i <= last; ++i) {
 ForkJoinTask<?> t = tasks[i];
 if (t != null) {
 if (ex != null)
 t.cancel(false);
 else if ((t.doJoin() & ABNORMAL) != 0)
 ex = t.getException();
 }
 }
 if (ex != null)
 rethrow(ex);
 }

 /**
 * Forks all tasks in the specified collection, returning when
 * {@code isDone} holds for each task or an (unchecked) exception
 * is encountered, in which case the exception is rethrown. If
 * more than one task encounters an exception, then this method
 * throws any one of these exceptions. If any task encounters an
 * exception, others may be cancelled. However, the execution
 * status of individual tasks is not guaranteed upon exceptional
 * return. The status of each task may be obtained using {@link
 * #getException()} and related methods to check if they have been
 * cancelled, completed normally or exceptionally, or left
 * unprocessed.
 *
 * @param tasks the collection of tasks
 * @param <T> the type of the values returned from the tasks
 * @return the tasks argument, to simplify usage
 * @throws NullPointerException if tasks or any element are null
 */
 public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) {
 if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) {
 invokeAll(tasks.toArray(new ForkJoinTask<?>[0]));
 return tasks;
 }
 @SuppressWarnings("unchecked")
 List<? extends ForkJoinTask<?>> ts =
 (List<? extends ForkJoinTask<?>>) tasks;
 Throwable ex = null;
 int last = ts.size() - 1;
 for (int i = last; i >= 0; --i) {
 ForkJoinTask<?> t = ts.get(i);
 if (t == null) {
 if (ex == null)
 ex = new NullPointerException();
 }
 else if (i != 0)
 t.fork();
 else if ((t.doInvoke() & ABNORMAL) != 0 && ex == null)
 ex = t.getException();
 }
 for (int i = 1; i <= last; ++i) {
 ForkJoinTask<?> t = ts.get(i);
 if (t != null) {
 if (ex != null)
 t.cancel(false);
 else if ((t.doJoin() & ABNORMAL) != 0)
 ex = t.getException();
 }
 }
 if (ex != null)
 rethrow(ex);
 return tasks;
 }

 /**
 * Attempts to cancel execution of this task. This attempt will
 * fail if the task has already completed or could not be
 * cancelled for some other reason. If successful, and this task
 * has not started when {@code cancel} is called, execution of
 * this task is suppressed. After this method returns
 * successfully, unless there is an intervening call to {@link
 * #reinitialize}, subsequent calls to {@link #isCancelled},
 * {@link #isDone}, and {@code cancel} will return {@code true}
 * and calls to {@link #join} and related methods will result in
 * {@code CancellationException}.
 *
 * <p>This method may be overridden in subclasses, but if so, must
 * still ensure that these properties hold. In particular, the
 * {@code cancel} method itself must not throw exceptions.
 *
 * <p>This method is designed to be invoked by <em>other</em>
 * tasks. To terminate the current task, you can just return or
 * throw an unchecked exception from its computation method, or
 * invoke {@link #completeExceptionally(Throwable)}.
 *
 * @param mayInterruptIfRunning this value has no effect in the
 * default implementation because interrupts are not used to
 * control cancellation.
 *
 * @return {@code true} if this task is now cancelled
 */
 public boolean cancel(boolean mayInterruptIfRunning) {
 int s = abnormalCompletion(DONE | ABNORMAL);
 return (s & (ABNORMAL | THROWN)) == ABNORMAL;
 }

 public final boolean isDone() {
 return status < 0;
 }

 public final boolean isCancelled() {
 return (status & (ABNORMAL | THROWN)) == ABNORMAL;
 }

 /**
 * Returns {@code true} if this task threw an exception or was cancelled.
 *
 * @return {@code true} if this task threw an exception or was cancelled
 */
 public final boolean isCompletedAbnormally() {
 return (status & ABNORMAL) != 0;
 }

 /**
 * Returns {@code true} if this task completed without throwing an
 * exception and was not cancelled.
 *
 * @return {@code true} if this task completed without throwing an
 * exception and was not cancelled
 */
 public final boolean isCompletedNormally() {
 return (status & (DONE | ABNORMAL)) == DONE;
 }

 /**
 * Returns the exception thrown by the base computation, or a
 * {@code CancellationException} if cancelled, or {@code null} if
 * none or if the method has not yet completed.
 *
 * @return the exception, or {@code null} if none
 */
 public final Throwable getException() {
 int s = status;
 return ((s & ABNORMAL) == 0 ? null :
 (s & THROWN) == 0 ? new CancellationException() :
 getThrowableException());
 }

 /**
 * Completes this task abnormally, and if not already aborted or
 * cancelled, causes it to throw the given exception upon
 * {@code join} and related operations. This method may be used
 * to induce exceptions in asynchronous tasks, or to force
 * completion of tasks that would not otherwise complete. Its use
 * in other situations is discouraged. This method is
 * overridable, but overridden versions must invoke {@code super}
 * implementation to maintain guarantees.
 *
 * @param ex the exception to throw. If this exception is not a
 * {@code RuntimeException} or {@code Error}, the actual exception
 * thrown will be a {@code RuntimeException} with cause {@code ex}.
 */
 public void completeExceptionally(Throwable ex) {
 setExceptionalCompletion((ex instanceof RuntimeException) ||
 (ex instanceof Error) ? ex :
 new RuntimeException(ex));
 }

 /**
 * Completes this task, and if not already aborted or cancelled,
 * returning the given value as the result of subsequent
 * invocations of {@code join} and related operations. This method
 * may be used to provide results for asynchronous tasks, or to
 * provide alternative handling for tasks that would not otherwise
 * complete normally. Its use in other situations is
 * discouraged. This method is overridable, but overridden
 * versions must invoke {@code super} implementation to maintain
 * guarantees.
 *
 * @param value the result value for this task
 */
 public void complete(V value) {
 try {
 setRawResult(value);
 } catch (Throwable rex) {
 setExceptionalCompletion(rex);
 return;
 }
 setDone();
 }

 /**
 * Completes this task normally without setting a value. The most
 * recent value established by {@link #setRawResult} (or {@code
 * null} by default) will be returned as the result of subsequent
 * invocations of {@code join} and related operations.
 *
 * @since 1.8
 */
 public final void quietlyComplete() {
 setDone();
 }

 /**
 * Waits if necessary for the computation to complete, and then
 * retrieves its result.
 *
 * @return the computed result
 * @throws CancellationException if the computation was cancelled
 * @throws ExecutionException if the computation threw an
 * exception
 * @throws InterruptedException if the current thread is not a
 * member of a ForkJoinPool and was interrupted while waiting
 */
 public final V get() throws InterruptedException, ExecutionException {
 int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
 doJoin() : externalInterruptibleAwaitDone();
 if ((s & THROWN) != 0)
 throw new ExecutionException(getThrowableException());
 else if ((s & ABNORMAL) != 0)
 throw new CancellationException();
 else
 return getRawResult();
 }

 /**
 * Waits if necessary for at most the given time for the computation
 * to complete, and then retrieves its result, if available.
 *
 * @param timeout the maximum time to wait
 * @param unit the time unit of the timeout argument
 * @return the computed result
 * @throws CancellationException if the computation was cancelled
 * @throws ExecutionException if the computation threw an
 * exception
 * @throws InterruptedException if the current thread is not a
 * member of a ForkJoinPool and was interrupted while waiting
 * @throws TimeoutException if the wait timed out
 */
 public final V get(long timeout, TimeUnit unit)
 throws InterruptedException, ExecutionException, TimeoutException {
 int s;
 long nanos = unit.toNanos(timeout);
 if (Thread.interrupted())
 throw new InterruptedException();
 if ((s = status) >= 0 && nanos > 0L) {
 long d = System.nanoTime() + nanos;
 long deadline = (d == 0L) ? 1L : d; // avoid 0
 Thread t = Thread.currentThread();
 if (t instanceof ForkJoinWorkerThread) {
 ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
 s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
 }
 else if ((s = ((this instanceof CountedCompleter) ?
 ForkJoinPool.common.externalHelpComplete(
 (CountedCompleter<?>)this, 0) :
 ForkJoinPool.common.tryExternalUnpush(this) ?
 doExec() : 0)) >= 0) {
 long ns, ms; // measure in nanosecs, but wait in millisecs
 while ((s = status) >= 0 &&
 (ns = deadline - System.nanoTime()) > 0L) {
 if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
 (s = (int)STATUS.getAndBitwiseOr(this, SIGNAL)) >= 0) {
 synchronized (this) {
 if (status >= 0)
 wait(ms); // OK to throw InterruptedException
 else
 notifyAll();
 }
 }
 }
 }
 }
 if (s >= 0)
 throw new TimeoutException();
 else if ((s & THROWN) != 0)
 throw new ExecutionException(getThrowableException());
 else if ((s & ABNORMAL) != 0)
 throw new CancellationException();
 else
 return getRawResult();
 }

 /**
 * Joins this task, without returning its result or throwing its
 * exception. This method may be useful when processing
 * collections of tasks when some have been cancelled or otherwise
 * known to have aborted.
 */
 public final void quietlyJoin() {
 doJoin();
 }

 /**
 * Commences performing this task and awaits its completion if
 * necessary, without returning its result or throwing its
 * exception.
 */
 public final void quietlyInvoke() {
 doInvoke();
 }

 /**
 * Possibly executes tasks until the pool hosting the current task
 * {@linkplain ForkJoinPool#isQuiescent is quiescent}. This
 * method may be of use in designs in which many tasks are forked,
 * but none are explicitly joined, instead executing them until
 * all are processed.
 */
 public static void helpQuiesce() {
 Thread t;
 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
 ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
 wt.pool.helpQuiescePool(wt.workQueue);
 }
 else
 ForkJoinPool.quiesceCommonPool();
 }

 /**
 * Resets the internal bookkeeping state of this task, allowing a
 * subsequent {@code fork}. This method allows repeated reuse of
 * this task, but only if reuse occurs when this task has either
 * never been forked, or has been forked, then completed and all
 * outstanding joins of this task have also completed. Effects
 * under any other usage conditions are not guaranteed.
 * This method may be useful when executing
 * pre-constructed trees of subtasks in loops.
 *
 * <p>Upon completion of this method, {@code isDone()} reports
 * {@code false}, and {@code getException()} reports {@code
 * null}. However, the value returned by {@code getRawResult} is
 * unaffected. To clear this value, you can invoke {@code
 * setRawResult(null)}.
 */
 public void reinitialize() {
 if ((status & THROWN) != 0)
 clearExceptionalCompletion();
 else
 status = 0;
 }

 /**
 * Returns the pool hosting the current thread, or {@code null}
 * if the current thread is executing outside of any ForkJoinPool.
 *
 * <p>This method returns {@code null} if and only if {@link
 * #inForkJoinPool} returns {@code false}.
 *
 * @return the pool, or {@code null} if none
 */
 public static ForkJoinPool getPool() {
 Thread t = Thread.currentThread();
 return (t instanceof ForkJoinWorkerThread) ?
 ((ForkJoinWorkerThread) t).pool : null;
 }

 /**
 * Returns {@code true} if the current thread is a {@link
 * ForkJoinWorkerThread} executing as a ForkJoinPool computation.
 *
 * @return {@code true} if the current thread is a {@link
 * ForkJoinWorkerThread} executing as a ForkJoinPool computation,
 * or {@code false} otherwise
 */
 public static boolean inForkJoinPool() {
 return Thread.currentThread() instanceof ForkJoinWorkerThread;
 }

 /**
 * Tries to unschedule this task for execution. This method will
 * typically (but is not guaranteed to) succeed if this task is
 * the most recently forked task by the current thread, and has
 * not commenced executing in another thread. This method may be
 * useful when arranging alternative local processing of tasks
 * that could have been, but were not, stolen.
 *
 * @return {@code true} if unforked
 */
 public boolean tryUnfork() {
 Thread t;
 return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
 ForkJoinPool.common.tryExternalUnpush(this));
 }

 /**
 * Returns an estimate of the number of tasks that have been
 * forked by the current worker thread but not yet executed. This
 * value may be useful for heuristic decisions about whether to
 * fork other tasks.
 *
 * @return the number of tasks
 */
 public static int getQueuedTaskCount() {
 Thread t; ForkJoinPool.WorkQueue q;
 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
 q = ((ForkJoinWorkerThread)t).workQueue;
 else
 q = ForkJoinPool.commonSubmitterQueue();
 return (q == null) ? 0 : q.queueSize();
 }

 /**
 * Returns an estimate of how many more locally queued tasks are
 * held by the current worker thread than there are other worker
 * threads that might steal them, or zero if this thread is not
 * operating in a ForkJoinPool. This value may be useful for
 * heuristic decisions about whether to fork other tasks. In many
 * usages of ForkJoinTasks, at steady state, each worker should
 * aim to maintain a small constant surplus (for example, 3) of
 * tasks, and to process computations locally if this threshold is
 * exceeded.
 *
 * @return the surplus number of tasks, which may be negative
 */
 public static int getSurplusQueuedTaskCount() {
 return ForkJoinPool.getSurplusQueuedTaskCount();
 }

 // Extension methods

 /**
 * Returns the result that would be returned by {@link #join}, even
 * if this task completed abnormally, or {@code null} if this task
 * is not known to have been completed. This method is designed
 * to aid debugging, as well as to support extensions. Its use in
 * any other context is discouraged.
 *
 * @return the result, or {@code null} if not completed
 */
 public abstract V getRawResult();

 /**
 * Forces the given value to be returned as a result. This method
 * is designed to support extensions, and should not in general be
 * called otherwise.
 *
 * @param value the value
 */
 protected abstract void setRawResult(V value);

 /**
 * Immediately performs the base action of this task and returns
 * true if, upon return from this method, this task is guaranteed
 * to have completed normally. This method may return false
 * otherwise, to indicate that this task is not necessarily
 * complete (or is not known to be complete), for example in
 * asynchronous actions that require explicit invocations of
 * completion methods. This method may also throw an (unchecked)
 * exception to indicate abnormal exit. This method is designed to
 * support extensions, and should not in general be called
 * otherwise.
 *
 * @return {@code true} if this task is known to have completed normally
 */
 protected abstract boolean exec();

 /**
 * Returns, but does not unschedule or execute, a task queued by
 * the current thread but not yet executed, if one is immediately
 * available. There is no guarantee that this task will actually
 * be polled or executed next. Conversely, this method may return
 * null even if a task exists but cannot be accessed without
 * contention with other threads. This method is designed
 * primarily to support extensions, and is unlikely to be useful
 * otherwise.
 *
 * @return the next task, or {@code null} if none are available
 */
 protected static ForkJoinTask<?> peekNextLocalTask() {
 Thread t; ForkJoinPool.WorkQueue q;
 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
 q = ((ForkJoinWorkerThread)t).workQueue;
 else
 q = ForkJoinPool.commonSubmitterQueue();
 return (q == null) ? null : q.peek();
 }

 /**
 * Unschedules and returns, without executing, the next task
 * queued by the current thread but not yet executed, if the
 * current thread is operating in a ForkJoinPool. This method is
 * designed primarily to support extensions, and is unlikely to be
 * useful otherwise.
 *
 * @return the next task, or {@code null} if none are available
 */
 protected static ForkJoinTask<?> pollNextLocalTask() {
 Thread t;
 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
 null;
 }

 /**
 * If the current thread is operating in a ForkJoinPool,
 * unschedules and returns, without executing, the next task
 * queued by the current thread but not yet executed, if one is
 * available, or if not available, a task that was forked by some
 * other thread, if available. Availability may be transient, so a
 * {@code null} result does not necessarily imply quiescence of
 * the pool this task is operating in. This method is designed
 * primarily to support extensions, and is unlikely to be useful
 * otherwise.
 *
 * @return a task, or {@code null} if none are available
 */
 protected static ForkJoinTask<?> pollTask() {
 Thread t; ForkJoinWorkerThread wt;
 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
 null;
 }

 /**
 * If the current thread is operating in a ForkJoinPool,
 * unschedules and returns, without executing, a task externally
 * submitted to the pool, if one is available. Availability may be
 * transient, so a {@code null} result does not necessarily imply
 * quiescence of the pool. This method is designed primarily to
 * support extensions, and is unlikely to be useful otherwise.
 *
 * @return a task, or {@code null} if none are available
 * @since 9
 */
 protected static ForkJoinTask<?> pollSubmission() {
 Thread t;
 return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
 ((ForkJoinWorkerThread)t).pool.pollSubmission() : null;
 }

 // tag operations

 /**
 * Returns the tag for this task.
 *
 * @return the tag for this task
 * @since 1.8
 */
 public final short getForkJoinTaskTag() {
 return (short)status;
 }

 /**
 * Atomically sets the tag value for this task and returns the old value.
 *
 * @param newValue the new tag value
 * @return the previous value of the tag
 * @since 1.8
 */
 public final short setForkJoinTaskTag(short newValue) {
 for (int s;;) {
 if (STATUS.weakCompareAndSet(this, s = status,
 (s & ~SMASK) | (newValue & SMASK)))
 return (short)s;
 }
 }

 /**
 * Atomically conditionally sets the tag value for this task.
 * Among other applications, tags can be used as visit markers
 * in tasks operating on graphs, as in methods that check: {@code
 * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
 * before processing, otherwise exiting because the node has
 * already been visited.
 *
 * @param expect the expected tag value
 * @param update the new tag value
 * @return {@code true} if successful; i.e., the current value was
 * equal to {@code expect} and was changed to {@code update}.
 * @since 1.8
 */
 public final boolean compareAndSetForkJoinTaskTag(short expect, short update) {
 for (int s;;) {
 if ((short)(s = status) != expect)
 return false;
 if (STATUS.weakCompareAndSet(this, s,
 (s & ~SMASK) | (update & SMASK)))
 return true;
 }
 }

 /**
 * Adapter for Runnables. This implements RunnableFuture
 * to be compliant with AbstractExecutorService constraints
 * when used in ForkJoinPool.
 */
 static final class AdaptedRunnable<T> extends ForkJoinTask<T>
 implements RunnableFuture<T> {
 final Runnable runnable;
 T result;
 AdaptedRunnable(Runnable runnable, T result) {
 if (runnable == null) throw new NullPointerException();
 this.runnable = runnable;
 this.result = result; // OK to set this even before completion
 }
 public final T getRawResult() { return result; }
 public final void setRawResult(T v) { result = v; }
 public final boolean exec() { runnable.run(); return true; }
 public final void run() { invoke(); }
 public String toString() {
 return super.toString() + "[Wrapped task = " + runnable + "]";
 }
 private static final long serialVersionUID = 5232453952276885070L;
 }

 /**
 * Adapter for Runnables without results.
 */
 static final class AdaptedRunnableAction extends ForkJoinTask<Void>
 implements RunnableFuture<Void> {
 final Runnable runnable;
 AdaptedRunnableAction(Runnable runnable) {
 if (runnable == null) throw new NullPointerException();
 this.runnable = runnable;
 }
 public final Void getRawResult() { return null; }
 public final void setRawResult(Void v) { }
 public final boolean exec() { runnable.run(); return true; }
 public final void run() { invoke(); }
 public String toString() {
 return super.toString() + "[Wrapped task = " + runnable + "]";
 }
 private static final long serialVersionUID = 5232453952276885070L;
 }

 /**
 * Adapter for Runnables in which failure forces worker exception.
 */
 static final class RunnableExecuteAction extends ForkJoinTask<Void> {
 final Runnable runnable;
 RunnableExecuteAction(Runnable runnable) {
 if (runnable == null) throw new NullPointerException();
 this.runnable = runnable;
 }
 public final Void getRawResult() { return null; }
 public final void setRawResult(Void v) { }
 public final boolean exec() { runnable.run(); return true; }
 void internalPropagateException(Throwable ex) {
 rethrow(ex); // rethrow outside exec() catches.
 }
 private static final long serialVersionUID = 5232453952276885070L;
 }

 /**
 * Adapter for Callables.
 */
 static final class AdaptedCallable<T> extends ForkJoinTask<T>
 implements RunnableFuture<T> {
 final Callable<? extends T> callable;
 T result;
 AdaptedCallable(Callable<? extends T> callable) {
 if (callable == null) throw new NullPointerException();
 this.callable = callable;
 }
 public final T getRawResult() { return result; }
 public final void setRawResult(T v) { result = v; }
 public final boolean exec() {
 try {
 result = callable.call();
 return true;
 } catch (RuntimeException rex) {
 throw rex;
 } catch (Exception ex) {
 throw new RuntimeException(ex);
 }
 }
 public final void run() { invoke(); }
 public String toString() {
 return super.toString() + "[Wrapped task = " + callable + "]";
 }
 private static final long serialVersionUID = 2838392045355241008L;
 }

 /**
 * Returns a new {@code ForkJoinTask} that performs the {@code run}
 * method of the given {@code Runnable} as its action, and returns
 * a null result upon {@link #join}.
 *
 * @param runnable the runnable action
 * @return the task
 */
 public static ForkJoinTask<?> adapt(Runnable runnable) {
 return new AdaptedRunnableAction(runnable);
 }

 /**
 * Returns a new {@code ForkJoinTask} that performs the {@code run}
 * method of the given {@code Runnable} as its action, and returns
 * the given result upon {@link #join}.
 *
 * @param runnable the runnable action
 * @param result the result upon completion
 * @param <T> the type of the result
 * @return the task
 */
 public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) {
 return new AdaptedRunnable<T>(runnable, result);
 }

 /**
 * Returns a new {@code ForkJoinTask} that performs the {@code call}
 * method of the given {@code Callable} as its action, and returns
 * its result upon {@link #join}, translating any checked exceptions
 * encountered into {@code RuntimeException}.
 *
 * @param callable the callable action
 * @param <T> the type of the callable's result
 * @return the task
 */
 public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) {
 return new AdaptedCallable<T>(callable);
 }

 // Serialization support

 private static final long serialVersionUID = -7721805057305804111L;

 /**
 * Saves this task to a stream (that is, serializes it).
 *
 * @param s the stream
 * @throws java.io.IOException if an I/O error occurs
 * @serialData the current run status and the exception thrown
 * during execution, or {@code null} if none
 */
 private void writeObject(java.io.ObjectOutputStream s)
 throws java.io.IOException {
 s.defaultWriteObject();
 s.writeObject(getException());
 }

 /**
 * Reconstitutes this task from a stream (that is, deserializes it).
 * @param s the stream
 * @throws ClassNotFoundException if the class of a serialized object
 * could not be found
 * @throws java.io.IOException if an I/O error occurs
 */
 private void readObject(java.io.ObjectInputStream s)
 throws java.io.IOException, ClassNotFoundException {
 s.defaultReadObject();
 Object ex = s.readObject();
 if (ex != null)
 setExceptionalCompletion((Throwable)ex);
 }

 // VarHandle mechanics
 private static final VarHandle STATUS;
 static {
 try {
 MethodHandles.Lookup l = MethodHandles.lookup();
 STATUS = l.findVarHandle(ForkJoinTask.class, "status", int.class);
 } catch (ReflectiveOperationException e) {
 throw new ExceptionInInitializerError(e);
 }
 }

}